This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Bovine leukemia virus (BLV) is a retrovirus that infects cattle. It is closely related to the human T cell leukemia virus. However, BLV mainly infects B cells, rather than T cells. BLV causes a B cell lymphosarcoma in about 5% of infected animals, but can also cause persistent lymphocytosis and other health problems in a larger percentage of infected cattle. In previous publications, Isaacson et al. reported that cattle experimentally infected with BLV had increased antibody responses to various non-BLV antigens, and a significantly increased number of B cells from these animals were found to express MHC-II molecules. To date, incubation of BLV-negative bovine lymphosarcoma (BL-3) cells with cell-free supernatants from BLV-infected FLK-BLV cells has resulted in the BL-3 cells becoming PCR-positive for BLV. These results are currently being confirmed, and various methods for quantitation of BLV in FLK-BLV supernatants are being investigated. Thus far, attempts to determine BLV titers by a reverse transcriptase assay, as well as by a TCID50 assay using BLV-susceptible CC81 cells, have been unsuccessful, but efforts are continuing. In addition, the BLV-infected BL-3 cells will soon be assayed by flow cytometry to determine whether levels of MHC-II expression are increased following in vitro BLV infection.